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Newly discovered Kuiper Belt object may be as large as Pluto's moon

Astronomers from MIT, Lowell Observatory and the Large Binocular Telescope Observatory on Mount Graham, Ariz., have discovered an icy planetary body in the Kuiper Belt roughly equal in size to Pluto's moon, Charon.

This object is intrinsically the brightest Kuiper Belt object found so far, said Lowell Observatory director Robert Millis, leader of the survey team.

The exact diameter of the object dubbed 2001 KX76 depends on assumptions that astronomers make about how its brightness relates to its size. Traditional assumptions make it the biggest by a significant amount, while others make it larger by at least 5 percent.

2001 KX76 may have a diameter of approximately 788 miles, bigger than Ceres, the largest known asteroid.Pluto's moon, Charon, has an estimated diameter of 744 miles.

2001 KX76 was discovered in the course of the Deep Ecliptic Survey, a NASA-funded search for KBOs being conducted by the Lowell-MIT-LBT team using the National Science Foundation's telescopes at Kitt Peak National Observatory near Tucson, Ariz., and Cerro Tololo Inter-American Observatory in Chile.

The team, which includes James L. Elliot, professor of earth, atmospheric and planetary sciences at MIT and director of the Wallace Astrophysical Observatory, spotted 2001 KX76 in deep digital images of the southern sky taken with the 4-meter Blanco Telescope at Cerro Tololo on May 22. Elliot was working with Lawrence H. Wasserman of Lowell Observatory.


2001 KX76 is currently at a distance of just over 4 billion miles from the sun. Its orbit is inclined by approximately 20 degrees with respect to the orbital plane of the major planets, but the detailed shape of its orbit remains uncertain. Evidence suggests that the newly discovered KBO may be in an orbital resonance with Neptune, orbiting the sun three times for each time Neptune completes four orbits.

The brightness and colors of 2001 KX76 have been measured by Elliot, Susan Kern and David Osip, all of MIT, with the Raymond and Beverly Sackler Magellan Instant Camera (MagIC) on the 6.5-meter Magellan Telescope at Las Campanas Observatory in Chile.

The object has a distinctly reddish color typical of many primitive bodies in the outer solar system. "2001 KX76 is so exciting because it demonstrates that significant bodies remain to be discovered in the Kuiper Belt," Millis said. "We have every reason to believe that objects ranging up to planets as large or larger than Pluto are out there waiting to be found. Until the Kuiper Belt has been thoroughly explored, we cannot pretend to know the extent or the content of the solar system."

Since 1992, astronomers have found more than 400 KBOs, but tens of thousands probably remain to be discovered. These objects are believed to be remnants from the formation of the solar system, and consequently are among the most primitive and least-evolved objects available for study by planetary astronomers.

The survey team's research is supported by the NASA Planetary Astronomy Program through grants to Lowell Observatory in Flagstaff, Ariz., and MIT.

A version of this article appeared in MIT Tech Talk on July 18, 2001.

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